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	/*
	* Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	package java.lang.ref;

	import sun.misc.Cleaner;
	import sun.misc.JavaLangRefAccess;
	import sun.misc.SharedSecrets;

	/**
	* Abstract base class for reference objects. This class defines the
	* operations common to all reference objects. Because reference objects are
	* implemented in close cooperation with the garbage collector, this class may
	* not be subclassed directly.
	*
	* @author Mark Reinhold
	* @since 1.2
	*/

	public abstract class Reference<T> {

	/* A Reference instance is in one of four possible internal states:
	*
	* Active: Subject to special treatment by the garbage collector. Some
	* time after the collector detects that the reachability of the
	* referent has changed to the appropriate state, it changes the
	* instance's state to either Pending or Inactive, depending upon
	* whether or not the instance was registered with a queue when it was
	* created. In the former case it also adds the instance to the
	* pending-Reference list. Newly-created instances are Active.
	*
	* Pending: An element of the pending-Reference list, waiting to be
	* enqueued by the Reference-handler thread. Unregistered instances
	* are never in this state.
	*
	* Enqueued: An element of the queue with which the instance was
	* registered when it was created. When an instance is removed from
	* its ReferenceQueue, it is made Inactive. Unregistered instances are
	* never in this state.
	*
	* Inactive: Nothing more to do. Once an instance becomes Inactive its
	* state will never change again.
	*
	* The state is encoded in the queue and next fields as follows:
	*
	* Active: queue = ReferenceQueue with which instance is registered, or
	* ReferenceQueue.NULL if it was not registered with a queue; next =
	* null.
	*
	* Pending: queue = ReferenceQueue with which instance is registered;
	* next = this
	*
	* Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
	* in queue, or this if at end of list.
	*
	* Inactive: queue = ReferenceQueue.NULL; next = this.
	*
	* With this scheme the collector need only examine the next field in order
	* to determine whether a Reference instance requires special treatment: If
	* the next field is null then the instance is active; if it is non-null,
	* then the collector should treat the instance normally.
	*
	* To ensure that a concurrent collector can discover active Reference
	* objects without interfering with application threads that may apply
	* the enqueue() method to those objects, collectors should link
	* discovered objects through the discovered field. The discovered
	* field is also used for linking Reference objects in the pending list.
	*/

	private T referent; /* Treated specially by GC */

	volatile ReferenceQueue<? super T> queue;

	/* When active: NULL
	* pending: this
	* Enqueued: next reference in queue (or this if last)
	* Inactive: this
	*/
	@SuppressWarnings("rawtypes")
	volatile Reference next;

	/* When active: next element in a discovered reference list maintained by GC (or this if last)
	* pending: next element in the pending list (or null if last)
	* otherwise: NULL
	*/
	transient private Reference<T> discovered; /* used by VM */


	/* Object used to synchronize with the garbage collector. The collector
	* must acquire this lock at the beginning of each collection cycle. It is
	* therefore critical that any code holding this lock complete as quickly
	* as possible, allocate no new objects, and avoid calling user code.
	*/
	static private class Lock { }
	private static Lock lock = new Lock();


	/* List of References waiting to be enqueued. The collector adds
	* References to this list, while the Reference-handler thread removes
	* them. This list is protected by the above lock object. The
	* list uses the discovered field to link its elements.
	*/
	private static Reference<Object> pending = null;

	/* High-priority thread to enqueue pending References
	*/
	private static class ReferenceHandler extends Thread {

	private static void ensureClassInitialized(Class<?> clazz) {
	try {
	Class.forName(clazz.getName(), true, clazz.getClassLoader());
	} catch (ClassNotFoundException e) {
	throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
	}
	}

	static {
	// pre-load and initialize InterruptedException and Cleaner classes
	// so that we don't get into trouble later in the run loop if there's
	// memory shortage while loading/initializing them lazily.
	ensureClassInitialized(InterruptedException.class);
	ensureClassInitialized(Cleaner.class);
	}

	ReferenceHandler(ThreadGroup g, String name) {
	super(g, name);
	}

	public void run() {
	while (true) {
	tryHandlePending(true);
	}
	}
	}

	/**
	* Try handle pending {@link Reference} if there is one.<p>
	* Return {@code true} as a hint that there might be another
	* {@link Reference} pending or {@code false} when there are no more pending
	* {@link Reference}s at the moment and the program can do some other
	* useful work instead of looping.
	*
	* @param waitForNotify if {@code true} and there was no pending
	* {@link Reference}, wait until notified from VM
	* or interrupted; if {@code false}, return immediately
	* when there is no pending {@link Reference}.
	* @return {@code true} if there was a {@link Reference} pending and it
	* was processed, or we waited for notification and either got it
	* or thread was interrupted before being notified;
	* {@code false} otherwise.
	*/
	static boolean tryHandlePending(boolean waitForNotify) {
	Reference<Object> r;
	Cleaner c;
	try {
	synchronized (lock) {
	if (pending != null) {
	r = pending;
	// 'instanceof' might throw OutOfMemoryError sometimes
	// so do this before un-linking 'r' from the 'pending' chain...
	c = r instanceof Cleaner ? (Cleaner) r : null;
	// unlink 'r' from 'pending' chain
	pending = r.discovered;
	r.discovered = null;
	} else {
	// The waiting on the lock may cause an OutOfMemoryError
	// because it may try to allocate exception objects.
	if (waitForNotify) {
	lock.wait();
	}
	// retry if waited
	return waitForNotify;
	}
	}
	} catch (OutOfMemoryError x) {
	// Give other threads CPU time so they hopefully drop some live references
	// and GC reclaims some space.
	// Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
	// persistently throws OOME for some time...
	Thread.yield();
	// retry
	return true;
	} catch (InterruptedException x) {
	// retry
	return true;
	}

	// Fast path for cleaners
	if (c != null) {
	c.clean();
	return true;
	}

	ReferenceQueue<? super Object> q = r.queue;
	if (q != ReferenceQueue.NULL) q.enqueue(r);
	return true;
	}

	static {
	ThreadGroup tg = Thread.currentThread().getThreadGroup();
	for (ThreadGroup tgn = tg;
	tgn != null;
	tg = tgn, tgn = tg.getParent());
	Thread handler = new ReferenceHandler(tg, "Reference Handler");
	/* If there were a special system-only priority greater than
	* MAX_PRIORITY, it would be used here
	*/
	handler.setPriority(Thread.MAX_PRIORITY);
	handler.setDaemon(true);
	handler.start();

	// provide access in SharedSecrets
	SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
	@Override
	public boolean tryHandlePendingReference() {
	return tryHandlePending(false);
	}
	});
	}

	/* -- Referent accessor and setters -- */

	/**
	* Returns this reference object's referent. If this reference object has
	* been cleared, either by the program or by the garbage collector, then
	* this method returns <code>null</code>.
	*
	* @return The object to which this reference refers, or
	* <code>null</code> if this reference object has been cleared
	*/
	public T get() {
	return this.referent;
	}

	/**
	* Clears this reference object. Invoking this method will not cause this
	* object to be enqueued.
	*
	* <p> This method is invoked only by Java code; when the garbage collector
	* clears references it does so directly, without invoking this method.
	*/
	public void clear() {
	this.referent = null;
	}


	/* -- Queue operations -- */

	/**
	* Tells whether or not this reference object has been enqueued, either by
	* the program or by the garbage collector. If this reference object was
	* not registered with a queue when it was created, then this method will
	* always return <code>false</code>.
	*
	* @return <code>true</code> if and only if this reference object has
	* been enqueued
	*/
	public boolean isEnqueued() {
	return (this.queue == ReferenceQueue.ENQUEUED);
	}

	/**
	* Adds this reference object to the queue with which it is registered,
	* if any.
	*
	* <p> This method is invoked only by Java code; when the garbage collector
	* enqueues references it does so directly, without invoking this method.
	*
	* @return <code>true</code> if this reference object was successfully
	* enqueued; <code>false</code> if it was already enqueued or if
	* it was not registered with a queue when it was created
	*/
	public boolean enqueue() {
	return this.queue.enqueue(this);
	}


	/* -- Constructors -- */

	Reference(T referent) {
	this(referent, null);
	}

	Reference(T referent, ReferenceQueue<? super T> queue) {
	this.referent = referent;
	this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
	}

	}

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